(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof, and in particular, to an impulsive driving liquid crystal display and a driving method thereof.
(b) Description of Related Art
A liquid crystal display (LCD) includes a pair of panels provided with field generating electrodes and a liquid crystal (LC) layer having dielectric anisotropy, which is disposed between the two panels. The field generating electrodes generally include a plurality of pixel electrodes arranged in a matrix and connected to switching elements such as thin film transistors (TFTs) to be supplied with data voltages every row and a common electrode covering an entire surface of a panel and supplied with a common voltage. A pair of field generating electrodes that generate the electric field in cooperation with each other and a liquid crystal disposed therebetween form so called a liquid crystal capacitor that is a basic element of a pixel along with a switching element.
The LCD applies the voltages to the field generating electrodes to generate electric field to the liquid crystal layer, and the strength of the electric field can be controlled by adjusting the voltage across the liquid crystal capacitor. Since the electric field determine the orientations of liquid crystal molecules and the molecular orientations determine the transmittance of light passing through the liquid crystal layer, the light transmittance is adjusted by controlling the applied voltages, thereby obtaining desired images.
In order to prevent image deterioration due to long-time application of the unidirectional electric field, etc., polarity of the data voltages with respect to the common voltage is reversed every frame, every row, or every pixel.
The polarity inversion of the data voltages increases the charging time of the liquid crystal capacitor since the response time of the liquid crystal is not so fast. Therefore, it takes long time for the liquid crystal capacitor to reach a target luminance (or target voltage) such that an image displayed by the LCD is unclear and blurred.
In order to solve this problem, impulsive driving that inserts a black image for a short time between normal images is developed.
The impulsive driving includes an impulsive emission type driving that periodically lights off a backlight lamp to yield black images and a cyclic resetting type driving that periodically applies a black data voltage for making the pixels in a black state to the pixels between the applications of normal data voltages.
However, these techniques do not compensate the large response time of the liquid crystal yet and the response time of the backlight lamp is large, too. Therefore, afterimages and flickering are generated to deteriorate image quality. In addition, the cyclic resetting type driving may decrease the time for applying normal data voltages for displaying normal images such that the liquid crystal capacitor do not reach a target luminance.
The decrease of the charging time for normal data voltages may be compensated by precharging the liquid crystal capacitor for a time to reduce the difference between the current luminance and the target luminance, thereby enabling to reach the target luminance for a given time.
In the meantime, the switching elements selectively transmit the data voltages for the liquid crystal capacitors in response to gate signals and thus the LCD includes gate lines for transmitting the gate signals and data lines for transmitting data voltages. The gate signal is a pulse-like signal including a gate-on voltage for turning on the switching elements and a gate-off voltage for turning off the switching elements and the gate signal is generated by a gate driver. The gate driver for a high resolution LCD may include a plurality of gate driving circuits, each gate driving circuit connected to a group of the gate lines. The gate-on voltage is sequentially outputted to the gate lines from a first gate driving circuit, and when the scanning of the gate-on voltage for the gate lines connected to the first gate driving circuit is finished, the first gate driving circuit sends a control signal to a next gate driving circuit to start the scanning of the gate-on voltage.
For precharging and impulsive driving, the gate signal further includes pulses for precharging and for impulsive charging, and the pulses of the gate signal are required to be appropriately arranged. In particular, it is preferable that the scanning of the pulses of the gate signal is smoothly passed between the gate driving circuits so that all the gate lines may transmit the gate signals in a uniform state and all the pixels may display under a uniform condition. When some pixels experience unsatisfied precharging, a transverse stripe may be generated on the positions of the LCD where such pixels are disposed.